Agreed, his concern was that he might be singled out for enhanced screening in the future based on this one incident.

And agreed, he was given the benefit of the doubt by the TSOs he dealt with, in that they believed his assertion that he had forgotten that he had a knife in his carry-on.

But "even to the extent of being offered the opportunity to mail the contraband item back to himself"?

TSOs have no legal authority to confiscate a prohibited item or to dispose of it without the owner's consent. Indeed, TSOs do not even have the legal authority to confiscate an illegal item such as narcotics. Their only authority in such matters is to prohibit the entry of persons possessing prohibited items into the sterile area, and to notify local law enforcement if they witness a traveler in possession of an illegal item.

Any attempt by a TSO to take a traveler's personal property, either to keep it for themselves or to dispose of it in the trash or to put it in the "confiscated items" pile, is theft. If a TSO finds a prohibited item during the course of a search, he is obligated to inform the owner of the item that they must dispose of the item before they will be allowed to enter the sterile area, by giving it to someone, mailing somewhere, leaving it in his vehicle (if available), or even disposing of the item in the trash. But the choice of method is completely up to the traveler (unless the item is actually illegal, in which case the TSO is obligated to report the illegal item to local LEOs).

"Even to the extent..." You make it sound like the TSO did him a favor by not relieving him of his knife without his consent.